That’s outdated info, v3 wasn’t removed, it was planned to be removed but one thing lead to another and Helix became a thing, and along with the announcement of v5 deprecation/removal the removal of v3 endpoints was pushed back to be in line with that of v5. The docs page shows the important dates https://dev.twitch.tv/docs/v5/
with the key thing being Note: The default version of the API is v3 until Dec. 31, 2018. However, both v3 and v5 are deprecated and will be removed altogether at the end of 2018.
As for the original issue, this is the 2nd time I’ve seen someone recently have 500 errors on the v3 version of that endpoint. I’m not sure what the issue is as I don’t use v3 myself any more, well, except from a few undocumented endpoints